Infrared analyzer and process control



May 20, 1958 E. c. MILLER 2,835,116

INFRARED ANALYZER AND PROCESS CONTROL Filed June 9, 1955 a I9 f 24 ACCUMULATOR u@ OVERHEAD PRoDUcTS FED lg l J i BOT'IQM PRODUCT ETHANE SAMPLE IN 3| 3525 [36 (33 F/G.3 Ar RNE United States Patent O 2,835,l 16 Patented May 20, 1958 ice 2,835,116 INFRARED ANALYZER AND PROCESS CONTROL Elmer C. Miller, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application June 9, 1955, Serial No. 514,345 Claims." (Cl. 62-175.5)

This invention relates to an infrared analyzer. In another aspect, it relates to a method of controlling an ethylene fractionator.

Where a stream containing ethylene and ethane is fractionated, to yield a high purity ethylene product, consid- It is a still further object to provide an analyzer and control system which is very efficient in operation, and resultsvin substantial economies processwise together with production of a product of high and consistent purity.

Various other objects, advantages and features of the invention will become apparent from the following detailed description taken in conjunction with the accompartying drawings, in which:

Figure l is a ow diagram of a process embodying the analyzer and control system of the invention;

Figure 2 is a schematic view of an analyzer constructed in accordance with the invention; and

Figure 3 is a graph illustrating a feature of the invention.

Referring now to Figure l, I have shown a fractionation column 10 to which a feed containing ethane and ethylene is fed through a line 11 under the control of a valve 12.

Suitable compositions for the feed, bottoms product erable ldifficulties are encountered in obtaining ei'lcient 20 and overhead are as follows:

Feed Bottom Product Overhead Product Specific Example Speclc Example Specific Example Mols Mol Mols Mol Mols Mol per percent Range per percent Range per percent Range day day day Methane... 13 2 Acetylene... 211 1.3 Ethylene-.- 13,096 80.0 Ethane-. 18.0 Propylene. 5

operation of the process with production of an overhead stream of consistent purity. It has been proposed to utilize an infrared analyzer sensitized to the ethylene concentration of the overhead'product. However, this has not proven satisfactory in practice because of the large concentration of ethylene in this product. Further, considerable interference is caused by thepresence of light gases in the overhead product.

In accordance with this invention, a sample is withdrawn from a tray of the fractionator near the top of the column and this sample is passed through an infrared analyzer which is sensitive to the ethane concentration rather than the ethylene concentration in the overhead product. In this manner, a very efficient and accurate control is obtained, and an overhead product of the desired degree of purity is continuously produced. Moreover, the accuracy of the control and analyses is not affected by the presence of light gases, such as methane, even when they are present in substantial amounts.

The analyzer is sensitized to the ethane concentration of the overhead product by placing a filter in one beam which has a strong absorption band at wavelengths of 3.4 to 3.5 microns. A suitable material for this filter is pure ethane. When the sample passes through both beams of the analyzer, variations in ethane content affect one beam but not the other so that the output of the instrument varies as a function of the ethane content. The output of the analyzer is advantageously utilized to control an operating Variable of the fractionation column such as the feed rate, reflux rate or heat supplied'to the kettle so as to maintain a predetermined ethylene content in the overhead product.

Accordingly, it is an object of the invention to 'provide an infrared analyzer of improved construction.

It is a still further object to provide a novel system for controlling a fractionation operation wherein a high purity ethylene stream is taken overhead.

Heat is supplied to the bottom of the column by a coil v 13 through which a heating medium such as steam circulated, the steam enteringby a line 14 incorporating a control valve 15. The bottoms product is withdrawn from the column through a line 16.

Overhead product is withdrawn from the column throughv a line 17 and passed through a condenser 18 to an'accumulator 1,9.

A portion of the overhead product frointhe accumulator 19 is returned as rcux to the top of the column through a line 20 under the control of a motor valve 21 while overhead product is Withdrawn from the accumulator through af line 22, incorporating a valve 23 connected to and controlled by a liquid level controller 24 having a sensing element in the accumulator 19.

Suitable operating conditions for the column are a temperature of 650 F. at the top, a bottom temperature of 20 F. and a pressure of 140 pounds per square inch gage.

In accordance with the invention, a sample is continuously withdrawn through a sample line 25 and fed to an infrared analyzer 26. As will be explained more fully hereinafter, the analyzer produces an output which is representative of the ethane content of the overhead product and, hence of its purity, The output can be advantageously utilized and control of the process by connecting it so as to control a process variable affecting the purity of the overhead product. For example, the output of the analyzer can be utilized to control the reflux rate by connecting it to valve 21, the feed rate by connecting it to valve 12 or theheat supplied to the bottom of the column by connecting it to valve 15.

The construction of the analyzer is shown in more detail by Figure 2 wherein it will be noted that infrared radiation from a source 30 is reflected by mirrors 31 and amature One @radiation beam passes through a filter 35 and a sample -cell 36 while 'the other beam 4lpasses through -a filter 37 and the sample cell .-36, these cells being provided with suitable windows transparent to the infrared radiation.

The detectors 33, 34 are `connected in a bridge circuit of va bridge and amplifier unit 38, lpreferably but not necessarily of -the Vtype Ashown lby Hutchins Patent 2,579,- 825, and the instrument can 'incorporate yautomatic standardization as shown in vsaid Hutchins patent. B y the operation of the bridge and amplifier circuits, the shaft of a motor 39 is caused to assume a position representative ofthe relative quantities Aof lradiation incident upon the detectors 33 and B4.

In accordance with the invention, the filter 35 --is constructed from a material which-has a strong absorption band at wavelengths -of 3.4 `to 3.5 microns which is a region where ethane absorbs radiation strongly. Advantageously, the filter should transmit lless than'l0% of the radiation in v-this wavelength band, preferably '5% lor less, and should be substantially transparent at adjoining wavelengths. To this end, the lter cell 3S can be filled `with ethane `which has the absorption characteristics indicated by the dashed line 40 of Figure 3. Also shown on the graph of -Figure 3 4by Ya solid line 41 are the absorption characteristics .of ethylene. The filter cell 37 can contain Vair or 'other transparent material.

Accordingly, when a sample of the overhead product in column is fed through the sample cell, -the intensity of both beams yis affected by changes in the ethylene concentration whereas changes in ethane content produce no .eiect on the upper -radiation beam, Valthough they do change -the intensity of the lower radiation beam. As a result, the position of motor 39 is representative of the ethane content of `the overhead product. Suprisingly, when the overhead product is vcontrolled so as to maintain a predetermined ethane concentration, an overhead product of consistent ethylene concentration and purity is readily produced, and .a much more uniform product is obtained than where Vthe analyzer is sensitized for ethylene.

Where Athe-sensitizing cell 35 is 1/z to l" long, andthe sample cell 36 is 4" long, a full-scale reading on a conventional recorder chart can Vbe readily obtained covering a percentage range of 0.0 to l1.5% ethane, thus lgiving a very sensitive control of the ethane concentration in the overhead product. Manifestly., it would be very diicult to obtaina full-scale .span over a range of ethylene content of 97 to 99%.

It is a further .feature of the analyzer and its accompanying control system that it is substantially insensitive to the presence of imethane -in the overhead product. Thus, the effect produced by methane upon the instrument reading is only 1/{50 of the change produced by ethane so that a variation of 2 or 3% in the methane content does not materially change the purity of the overhead product.

While the invention has been described in connection with a present, 'preferred embodiment, thereof, it is to be understood that this description is illustrative only and is not intended to limit the invention.

I clairn:

l. A method of controlling a fractionation process wherein a stream containing ethane and ethylene is fractionated to produce an overhead product containing 0.l to 3:0 mol percent et-hane, 0.0 'to 5.0 mol percent methane and the balance substantially all ethylene, wherein a process variable is ladjustable to control the purity of said overhead product, the steps which comprise continuously withdrawing a sample from the fractionation zone, passing twin beams of infrared radiation continuously through the withdrawn sample, passing one of said beams but not the other of said beams through a body of ethane, producing an output voltage representative of the relative intensities of said beams after they have passed through Asaid sample and said ethane, and controlling -said 'process variable in accordance with said voltage to maintain a constant purity of said overhead stream.

2. The process ofc'laim l wherein said process variable is 4thelrate of flow ofreiiux'of material to the fractionation zone.

3. The process of claim 1 wherein said process variable is the heat supplied to the bottom of the fractionation zone.

4. The process of claim l wherein said process variable is the feed rate to the fractionation zone.

5. A method of controlling a fractionation process wherein a stream containing ethane and ethylene is fractionated .to produce an overhead product containing 0.1 to 3.0 mol percent ethane, 0.0 to 10.0 mol percent methane and the balance substantially all ethylene, wherein a processivariable is adjustable to control the purity of said overhead product, the steps which comprise continuously withdrawing a sample from the fractionation zone, passing .twin beams of infrared radiation continuously through the withdrawn sample, passing one of said beams but-not `the other oflsaid beams through a body of ethanc, producing .an .output representative of the relative intensities-.of saidbeams after they have passed through said sample and said :ethane, and controlling said process variable in accordance with said output to maintain a constant purityof said overhead stream.

References Cited in the file of this patent UNITED STATES PATENTS 2,579,825 Hutchins Dec. 25, 1951 2,600,110 AHachmuth June .10, 1952 2,629,239 Gantt Feb. 24, 1953 2,697,789 Skarstrorn Dec. 2l, 1954 OTHER REFERENCES Elements of Fractional Distillation (fourth edition), by Robinsonand Gilliland, 'pages 472 to 476 relied on.

UNITED STATES PATENT OFFICE.

CERTIFICATE 0F 'CORRECTION Patent No., 2,835,116 May 20, 1958 Elmer C., Miller It is hereby certified 'that error appears in the-printed specifica-tion of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

o o Column 2, line 52, for n"-650 F." read -65 F. me; column 3,

line 35, for ".Suprisingly" read Surprisingly,

Signed and Sealed this 14th day of October 1958.,

(SEAL) Attest:

KARL H AXLINE ROBERT c. WATSON Attesting Officer Commissioner of Patents 

1. A METHOD OF CONTROLLING A FRACTIONATION PROCESS WHEREIN A STREAM CONTAINING ETHANE AND ETHYLENE IS FRACTIONATED TO PRODUCE AN OVERHEAD PRODUCT CONTAINING 0.1 TO 3.0 MOL PERCENT ETHANE, 0.0 TO 5.0 MOL PERCENT METHANE AND THE BALANCE SUBSTANTIALLY ALL ETHYLENE, WHEREIN A PROCESS VARIABLE IS ADJUSTABLE TO CONTROL THE PURITY OF SAID OVERHEAD PRODUCT, THE STEPS WHICH COMPRISE CONTINUOUSLY WITHDRAWING A SAMPLE FROM THE FRACTIONATION ZONE, PASSING TWIN BEAMS OF INFRARED RADIATION CONTINUOUSLY THROUGH THE WITHDRAWN SAMPLE, PASSING ONE OF SAID BEAMS BUT NOT THE OTHER OF SAID BEAMS THROUGH A BODY OF ETHANE, PRODUCING AN OUTPUT VOLTAGE REPRESENTATIVE OF THE RELATIVE INTENSITIES OF SAID BEAMS AFTER THEY HAVE PASSED THROUGH SAID SAMPLE AND SAID ETHANE, AND CONTROLLING SAID PROCESS VARIABLE IN ACCORDANCE WITH SAID VOLTAGE TO MAINTAIN A CONSTANT PURITY OF SAID OVERHEAD STREAM. 